The present invention is directed to a humidity measuring apparatus, and in particular an apparatus for measuring the moisture content of soil.
With the expected doubling of the world population in the next 40 years from the current level of 4.7 billion, food production will need to at least double to maintain the nutritional levels already considered inadequate in many countries. In many areas of the world, the availability of water resources and the cost of pumping irrigation water restricts food production. More efficient use of water is needed to meet the demand for increased food production.
One area where water resource efficiency can be improved is irrigation. Irrigation brings at least four potential benefits. It brings under cultivation arid lands, increases yields per acre per crop, allows more than one crop to be grown per year, and reduces risk of crop failure.
The soil parameter which controls whether or not the soil requires irrigation is the soil matric potential. A soil matric potential of 0 corresponds to soil completely saturated with water. Two important soil matric potential values with respect to plant growth are the field capacity at about -0.3 bar and the permanent wilting point at about -15 bars. At soil matric potentials from -0.3 bars to 0 bar the air channels in the soil are progressively filled with water. Since the roots need both oxygen and water for growth at soil matric potentials above -0.3 bar and close to zero the plant is under stress and in the worst case, i.e. soil matric potential of zero, the plant dies. As the soil matric potential drops below -0.3 bar, i.e. -0.3 bar to -15 bars, the plants must use more of their energy to extract water from the soil which places them under stress and limits growth and yield. At about -15 bars the roots can no longer extract water from the soil and the plant wilts and dies. The optimum soil matric potential for most crops and soil types is, therefore, from field capacity at about -0.3 bar to about -3 bars, below which most plants are under extreme stress. In many cases -0.3 bar to -1 bar is preferred.
A reliable soil moisture sensor should have high preferably from 0 to -15 bars. Optimum growth occurs close to field capacity (about -0.3 bar) but the grower should be able to let the field dry approaching the permanent wilting point without worrying about crop loss when either he is experiencing a water shortage, is close to harvest, or is trying to optimize his yield to water cost relationship.
It is been found difficult to develop a reliable soil moisture sensor in the range of -0.3 to -1 bar because these values correspond to a relative humidity in salt free soil of 99.98% and 99.93%, respectively. Any device designed to sense soil moisture levels should have a large response between 99.9 and 100% relative humidity.
A variety of techniques are currently used to determine soil moisture content, all of which have significant disadvantages. In one technique, an auger soil probe is drilled into the ground to withdraw a soil sample. The amount of moisture is estimated by feeling the withdrawn sample by hand, but for accurate measurement, the sample is sealed in plastic and sent to a laboratory. This probe method is time-consuming and laborious, particularly if it needs to be done daily over a large area.
Another technique to measure soil matric potential uses a tensiometer. In this technique, a porous cup filled with water is buried at the main level of the root zone. Water moves in or out of the cup in response to soil moisture level. A tube connects the cup to a vacuum gauge or a mercury manometer. The readings indicate the soil matric potential. Tensiometers are labor intensive, require readings in the field daily, and must be maintained with periodic additions of water. Further, they only cover the range of soil matric potential of 0 to about -0.8 bar.
Another technique utilizes electrical resistance blocks made of gypsum plaster of paris with two electrodes embedded in the block. The porous block is buried in the soil, and the wires from the electrodes are connected to a meter that gives a reading of the electrical resistance between the electrodes. The main problem with these blocks is that soil salinity affects the reading, and thus these blocks require extensive calibration and can degrade in some soil environments.
Another technique used is a neutron probe using a radioactive isotope, usually americium, to measure water content. Tubes are installed in the soil at the desired depth. The probe is inserted into the tube and the amount of back scatter from protons in the soil is measured. A problem with neutron probes is they must be calibrated for each soil type since they measure total water, some of which is not available water for plant growth, as it is chemically bound or otherwise not available to the plant. They also must be calibrated for soil depth because soil composition varies with depth. In addition, a neutron probe is expensive, costing in excess of $3,000, is radioactive, and must be used by a skilled technician.
Infrared sensors can be used to differentiate stressed plants from healthy plants because the temperature of the green leaves of stressed plants is higher than the temperature of leaves of healthy plants. A problem with infrared sensors is that they do not differentiate between water, salt, insect, and disease stresses. In addition, they are expensive, costing about $2,500.
Another measuring system used is a heat dissipation sensor, usually made of a ceramic body containing a heater and a thermocouple. The heat capacity of a ceramic body is a function of its water content which is proportional to the water content of the adjacent soil. In this technique, the heater in the ceramic body is activated for about one minute, and the temperature increase of the sensor is measured. The measured temperature increase is proportional to the heat capacity of the ceramic body, and therefore, the water content of the soil. Heat dissipation sensors have poor sensitivity, and are costly, requiring extensive calibration.
A variety of devices have been developed for automatically controlling irrigation which depend on the moisture content of the soil to be irrigated. For example, Ornstein in U.S. Pat. No. 4,182,357 describes a valve containing a water-swellable polymer that swells against a compressible water line, closing off the water line as the polymer swells. The Ornstein device is designed to control at a specific soil matric potential. For example, if it is designed for -0.3 bar or field capacity then water flows to the soil at soil matric potentials below -0.3 bar. At soil matric potentials above -0.3 bar water does not flow. The Ornstein device is essentially an on/off device designed for a specific soil matric potential value. A device that provides an indication of the moisture content of soil over a much wider range would be more valuable.
Several problems exist with this device. The first is that it requires a constant water supply, i.e. water pressure. If it does not have a constant water pressure it can dry out at soil matric potentials below -0.3 bar and when the water is turned on a flooding condition can occur. Another problem with the Ornstein device is that once the device is built for a given soil moisture potential, the device can not be varied. This takes the control out of the hands of the grower. The grower needs a system which will allow him to control field irrigation based upon his strategy on how to use his allotted irrigation water when it is available to him. He needs to sequence the irrigation of his field, determine how much to apply, and when to apply it. The data he needs include knowledge of soil moisture conditions over the range of soil matric potential from 0 to -15 bars.